1. Field of the Invention
This invention relates to a golf ball injection mold which permits a cover or intermediate layer of a uniform thickness to be formed around a core. It also relates to golf balls prepared using the mold.
2. Prior Art
As is well known, golf balls include balls of the wound type in which a rubber or resin cover stock is molded around a wound core formed by winding thread rubber around a center, and balls of the solid type in which a resin cover stock is molded around a solid core of solid elastomer.
Golf balls of the solid type (solid golf balls) were developed subsequent to golf balls of the wound type. In addition to the fact that solid golf balls essentially fly a longer distance when hit, improvements have been made on sold golf balls with respect to the control of the ball by the spin imparted thereto and the feel of the ball that the player gets when hitting the ball. Nowadays many players prefer the solid golf balls regardless of whether players are amateur or professional.
Injection molds are often used in preparing such solid golf balls, especially molding covers around cores. As shown in FIGS. 5A and 5B, a typical injection mold 1 defines a spherical cavity 2 having a plurality of dimple-forming projections (not shown) on its internal surface 2a. The spherical cavity 2 has an equator and opposed poles.
More particularly, the injection mold 1 includes a pair of upper and lower mold sections 1a and 1b which are removably mated along a parting plane or line P lying in substantial register with the equator of the cavity 2. A plurality of support pins 7 are disposed near each of the opposed poles of the cavity 2 for motion into and out of the cavity 2 for supporting a preformed solid core 3 at the center of the cavity 2. In FIG. 5, four support pins 7 are arranged near each pole, totaling eight support pins. An annular runner 4 nearly surrounding the cavity equator is provided in the mold 1. A plurality of (eight in FIG. 5) gates 5 equi-spaced along the equator, extend radially inward from the annular runner 4 and open to the cavity 2 for injecting a molding material into the cavity. With the solid core 3 supported at the cavity center by the support pins 7, a cover stock of thermoplastic resin (e.g., ionomer resin) is injected through the gates 5 into the space defined between the cavity surface 2a and the solid core 3, forming a cover around the core. The cover thus formed usually has a (radial) thickness of about 1.5 to 2.5 mm.
In FIGS. 5A and 5B, a main runner 6 provides fluid communication between the annular runner 4 and an injection molding machine. Vent holes 8 are drilled in the mold, and pins 9 are received in the holes 8 to define degassing gaps 10 therebetween, permitting gases and volatiles generated during molding to escape to the outside through the gaps 10.
When it is desired to mold a golf ball of a multilayer structure having an intermediate layer between a solid core and a cover, as in the molding of the cover, the intermediate layer may be formed by injecting a molding material such as a thermoplastic resin, followed by injection molding of a cover. In this case, the molding material is injected under a very high pressure of 1,000 to 3,000 kg/cm.sup.2. Often, the thicknesses of the intermediate layer and the cover are somewhat less than the thickness of the first described cover (formed without the intermediate layer).
When the cover or intermediate layer is formed around the solid core using the injection mold described above, portions of the molding material, typically thermoplastic resin, injected through the gates flow through the narrow space between the solid core and the cavity surface and eventually join together to form an integral cover. In this flow process, the resin portions from adjacent gates join each other in latitudinal directions of the cavity because of the relatively short distance between the gates, advance in longitudinal directions, and finally integrate together near the opposed poles.
In the above-described mold, however, the gates are merely arranged at equal intervals in a plane in registration with the cavity equator. The injected resin must flow from the gate position (corresponding to the equator) to the final integration position (corresponding to the opposed poles), that is, a distance equal to about a quarter of the cavity circumferential length. If the resin is somewhat poor flowing, welds or chicken feet can be formed near the opposed poles, resulting in molding defects.
When it is desired to form the cover (or intermediate layer) to a reduced thickness of 1 mm or less, the high injection pressure of the resin causes the solid core to be deformed into an oval shape like a rugby ball. As a result, the thickness of the cover is increased near the equator and reduced near the opposed poles. In some cases, the core can be off centered so that the core is exposed at the ball surface.